Remove default constructor lines that do nothing, and fix warnings with clang trunk...

Remove default constructor lines that do nothing, and fix warnings with clang trunk (#14300)

Summary:
The lines removed in this diff were no-op, but confusing: the default constructors in `store_handler.h` are implicitly deleted, since `std::runtime_error` has no default constructor.

Clang added a warning for this behavior [in September 2018](https://reviews.llvm.org/rL343285) (note that the warning is not just for cxx2a, despite the slightly confusing commit message), so building pytorch with a recent build of clang trunk causes spew of this warning, which is fixed by the present PR.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/14300

Differential Revision: D13260039

Pulled By: umanwizard

fbshipit-source-id: 92788dbd6794253e788ef26bde250a66d8fb917e